Research Of Isolated Hybrid Modular Multilevel DC/DC Converter

DC grid technology is an effective technical means to achieve grid-connected stability of new energy and improve power supply reliability.High-voltage and high-power DC/DC converter is one of the important foundations and key equipment of high-voltage direct current(HVDC)grid.This thesis studies the new topology,working characteristics as well as control strategy of Isolated Modular Multilevel DC/DC Converter(IMMDC)with module chain as generalized switch.First of all,this paper analyses the working mode of the module chain of the typical MMC-BUCK topology,and proposes that the module chain can be regarded as a generalized switch.Its external working characteristics and analysis methods are similar to those of traditional switches.On this basis,two IMMDC topologies are presented,including modular multi-level dual active bridge and isolated hybrid modular multi-level DC/DC converter(IHMMDC).Through comparative analysis,it is found that IHMMDC has lower voltage stress and better control freedom under the same conditions.This paper simplifies the IHMMDC topology and explains its power transmission mechanism via the analysis of DC and AC equivalent circuit.A dual-period control method of module chain is proposed to balance the DC bus capacitor voltage at the input side.On this basis,the working principle and characteristics of IHMMDC controlled by phase shifting and PWM are analyzed in detail,and the voltage balance control strategy of capacitor in submodule is proposed.The soft switching conditions of all switches are deduced based on the soft switching of a single HBSM.It owns a large phase shift range of soft switching when the duty cycle of PWM is larger than DC Voltage Conversion Ratio.In this paper,the design objectives of the prototype are input voltage(800V),output voltage(200V)and transmission power(3kW).Detailed parameters calculation and design process of each device of the converter are given.For complex control systems,this paper presents a three-layer control mode of main controller,auxiliary controller and modular controller.Finally,the prototype is built in the laboratory and verified by experiments.